Blender adding double forward slash (`//`) to image file paths on Mac
Turns out that the double forward slash is Blender's way of expressing relative file paths. E.g. ./file.jpg in a Unix filesystem is //file.jpg in Blender. So I needed to save my .blend file by turning off the default interpretation of filepaths as relative. 

Turns out that this modified question is already answered here: Absolute path of files in Blender (with Python).